1. Field of the Invention
The present invention relates to an improved program delivery control (PDC) system, that is, a system or method of transmitting labels identifying particular programs which are used to control domestic receiving equipment such as video cassette recorders.
2. Description of the Related Art
There exist already PDC methods in which program labels are transmitted frequently and, in some cases, at regular intervals, during the program. In the case of television, the labels may be carried by a dedicated line in the vertical blanking period (as in the German VPS system) or as part of the teletext service as, for example, the broadcast service data packet defined for CCIR system B teletext. Where the broadcast programs are radio programs, the labels may be carried by the RDS (radio data) system.
The transmitted labels are, as indicated above, used to control receiving equipment, in particular, video cassette recorders, which are caused to respond to particular program labels and to operate while those labels are being transmitted. In essence, a video cassette machine may record incoming programs only while predetermined labels are being transmitted. More sophisticated video recorders may also establish a `time window` during which the equipment may respond to a particular label and may incorporate a `flywheel` method of operation so that the occasional loss of a label in the received signal does not cause recording to be interrupted or to cut off prematurely.
One existing system, described in United Kingdom patent application No. 8821619 filed 15th Sep. 1988 describes a system of conditional access to a PDC service in which the program labels are varied continuously and systematically. Only equipment having a corresponding key and/or sequence generator can recognize the sequence of labels identifying a given program.
One problem that arises with such systems is that there may be an appreciable delay between detection of the first program label and commencement of actual operation of the video. This leads to the first few moments of a program not being recorded
European Patent Application No. 0214326 (Eitz) describes a system in which a `special` label is transmitted at a predetermined time before an upcoming change of program content. A similar system is described in Heller (German patent B328001). In both cases the `special` label is transmitted before the program change at regular intervals. These systems are, however, vulnerable to disruption by mis-transmitted signals. If, for example, a `special` label is missed, the start-up of the video recorder may be mistimed relative to the actual beginning of program because the first label after a break in the sequence may be treated as the first of the sequence, leading to late start-up of the recorder.